This invention relates to control circuits for a-c control elements in general and more particularly, to an improved control circuit for an a-c control element which consists of controlled rectifier elements and is connected to a transformer.
In an a-c control element, the current is controlled by appropriately delaying the flow of the current at the beginning of each half-wave. If the firing angles of the positive and negative half-waves do not agree exactly, than a d-c component is generated in the current. If the a-c control element is connected to a transformer, then the d-c component caused by unequal firing angles drives the preceding transformer unilaterally into saturation. This causes large magnetizing currents to flow, which increase the no-load losses of the transformer considerably.
Up to now, attempts have been made to prevent the generation of a d-c component through a very high angular accuracy of the control unit controlling the a-c control element. Such control units require a correspondingly large expenditure of money.